The invention relates to nonwoven fabrics and to processes for producing nonwoven fabrics. More specifically, the invention relates to nonwoven fabrics formed of hollow filaments and/or hollow fibers, useful for hygiene, medical, and industrial applications.
Nonwoven fabrics are widely used as components of nonwoven laminates for a variety of specific end uses. Nonwoven fabric laminates can be used in hygiene applications, such as disposable diapers, feminine hygiene products, incontinence pads, and the like; in medical applications, such as in disposable medical garments; and the like.
For example, disposable hygiene products, such as disposable diapers, typically include a liquid impermeable outer covering, an absorbent layer, and an inner layer which contacts the skin of the wearer. The liner layer can be a liquid permeable, porous nonwoven fabric, such as a carded web or a spunbonded web, and the outer covering can include a spunbonded or carded nonwoven fabric treated to impart barrier properties thereto, for example, by laminating the fabric to a polyolefin film.
A primary function of hygiene products is to contain body exudates to prevent soiling, wetting, or contamination of clothing or other articles. Hygiene products are also constructed to rapidly absorb waste, ideally permitting liquid to flow rapidly through the layer adjacent the wearer into the absorbent layer without permitting or facilitating re-transmission of liquid from the absorbent layer to the wearer side of the inner layer.
Other useful nonwoven laminate articles include barrier fabric which include one or more microfibrous layers, such as meltblown webs, sandwiched between outer spunbonded webs of continuous filaments. The barrier meltblown webs impede the passage of bacteria and other contaminants, and the outer spunbonded webs reinforce the inner webs and provide abrasion resistance.
Issues associated with the use of nonwoven fabrics in laminate applications include the ability of the laminate to contain waste, facilitate liquid strikethrough and minimize rewet, and provide barrier properties. This can be particularly problematic in hygiene products due to decreasing basis weights of nonwoven components to meet the increasing demands for thinner products.
Another issue in nonwoven laminate fabrics is the ability to provide the combination of contradictory properties, such as pleasing aesthetics (softness, flexibility, and the like) and strength and abrasion resistance in a single fabric. For example, spunbonded webs used in laminate products are typically formed of 100% polypropylene filaments to reinforce and protect inner layers from excessive stresses and potential damage during use. The resultant laminate can be more durable and have an improved appearance. This can also minimize contamination of sterile surfaces in medical applications by preventing loose fibers from contaminating sterile environments. However, polypropylene fabrics can suffer from poor aesthetics.
Fibrous webs formed of polyethylene can exhibit improved aesthetics relative to polypropylene fabrics, which make them desirable for incorporation into composite nonwoven fabrics. Nonetheless, in spite of these advantages, such fabrics suffer poor abrasion resistance, resulting in an unsightly fuzzed appearance and possible contamination in sterile environments.
The present invention provides nonwoven fabrics which can exhibit a combination of various properties, such as improved barrier and containment, good abrasion resistance and tensile strength, pleasing aesthetics, and the like. In one embodiment of the invention, the fabrics include a plurality of hollow continuous spunbonded filaments. In another embodiment of the invention, the fabrics include a plurality of hollow staple fibers. The hollow filaments and the hollow staple fibers are formed of a polypropylene composition. Exemplary polypropylene compositions include 100% polypropylene compositions and blends of polypropylene as the dominant or majority component with at least one other polymer.
The hollow spunbonded filaments and hollow staple fibers can impart several advantageous properties to the fabrics. For example, the use of hollow filaments and/or hollow staple fibers can allow the fabric manufacturer to increase the number of filaments and/or fibers in a fabric for a given basis weight, or conversely, to lower the basis weight of a fabric without lowering the number of filaments and/or fibers. This can improve the barrier properties of the fabrics without unduly increasing basis weight. This can also improve resistance to bleed through of adhesives and improved SAP containment and strikethrough/rewet for hygiene applications, due to the increased number of filaments and/or fibers for a given basis weight. The fabrics of the invention can also exhibit improved abrasion and similar or improved tensile and tear properties, as compared to heavier basis fabrics formed of solid filaments and/or fibers. Still further, the fabrics of the invention can exhibit improved (higher) opacity as compared to fabrics formed of solid filaments.
The fabrics can also exhibit abrasion resistance. This can increase durability and improve the appearance of disposable hygiene and other products. This can also minimize or prevent contamination of sterile environments due to loose fibers abraded from the surface of a nonwoven article in medical applications.
It is also currently believed that the hollow filaments and fibers can improve bonding properties of the fabrics. For example, it is believed that the fabrics of the invention can be thermally bonded to provide a cohesive fabric using lower bonding temperatures and/or a wider bonding window (i.e., wider range of bonding conditions). As a result, fabrics can be produced more readily which have superior strength and abrasion resistance.
For example, when comparing fabrics formed of solid fibers with the fabrics of the invention which include hollow fibers at a constant number of fibers per unit area and constant outer fiber diameter, the amount of material that has to be softened or melted to form the bonding points will be less for the hollow fibers. Thus, in a calender bonding process in which calender rolls are used to bond the nonwoven, the energy and pressure needed to compress and melt the bonding points will be less when hollow fibers are used. On the other hand, a nonwoven fabric which includes hollow fibers will have more fibers per unit area, as compared to a nonwoven fabric formed of solid fibers, at the same basis weight and with the same outer diameter of the fibers. This can give a stronger bonded material because more fibers can be tied down in the bonding points or sites.
Still further, the fabrics of the invention can exhibit desirable volumetric capacity. The volumetric capacity of a fabric can be defined as the portion of the total fabric volume not occupied by fibers and binders. Volumetric capacity can be important for storing, suspending or transporting other materials (gas, liquid or solid) by a nonwoven fabric. Fabrics made from hollow fibers can exhibit a higher volumetric capacity per unit of fabric weight and hence can be more efficient than conventional carrier fabrics made from solid fibers formed of the same material. For a typical thermally bonded nonwoven fabric, the ratio of volumetric capacity, Vc, to fabric mass, Mnw, can be expressed as:
Vc/Mnw=(t/Bwt)xe2x88x92(1/xcfx81)
wherein t=fabric thickness; Bwt=fabric basis weight; and p=density of fiber polymer type. Thus, for the ratio of void volume to fabric weight to increase for a given fabric construction, one must reduce basis weight while not effecting the fabric construction or geometry. The fabrics of the invention which include hollow fibers or filaments can achieve this improvement. Although the foregoing example applies to a homogenous, fibrous nonwoven fabric, the equation can be easily modified to include multi-component fibers as well as adhesive binders. Still further, the fabrics can exhibit improved (increased) capacity.
The hollow filaments and/or fibers are also believed to form stronger thermal bonds (in the fabric and in laminate structures) than comparable solid filaments and fibers. The thermal bonds can also have greater bond area relative to thermal bonds formed of comparable solid filaments or fibers. This can also improve fabric strength and abrasion resistance.
Further, by using hollow fibers, it is possible to increase the number of fibers for a given basis weight without sacrificing the loft of the nonwoven, due to the constant outer diameter of the hollow fibers or filaments. That is, with solid fibers, one can normally only increase the number of fibers for a given basis weight by reducing fiber outer diameter. However, decreased fiber diameter can result in reduced loft. The loft of a fabric can be important with regard to both aesthetic and physical properties of the fabric.